The invention relates to an arrangement for detecting motion of an encoder.
An apparatus for use in electronically controlled fuel injection systems is known from WO 90/06517 A1, which detects the speed, angular position and direction of movement of a rotation by using a single Hall sensor. The apparatus comprises a disc-shaped element which is fixedly connected to the camshaft of a combustion engine and is rotatable therewith. The disc-shaped element comprises a multitude of peripheral zones of substantially equal length, each of which has a first and a second area. A first part of the zones has first and second areas whose length clearly deviates from the first and second areas of a second part of the zones. Accordingly, the sensor supplies a signal whose frequency is directly related to the instantaneous speed of the disc-shaped element, but whose pulse width alternates in dependence upon the first and second parts of the zones. A program-controlled microprocessor detects the instantaneous angular position and the direction of rotation of the disc-shaped element by detecting the position of the second part of the zones and the sequence in which they are received.
Such an arrangement requires a very elaborate construction for the disc-shaped element. Moreover, an unambiguous determination of the speed, angular position and direction of movement of the rotation to be detected is only possible after approximately a full rotation of the disc-shaped element.
It is an object of the invention to provide an arrangement for detecting motion of an encoder, which allows a simple, fast and accurate measurement of both the speed and the detection of the direction of movement of this encoder.
According to the invention, this object is solved by an arrangement for detecting motion of an encoder which comprises magnetically active areas arranged along a main surface subtended at least substantially by a second spatial co-ordinate aligned in the direction of movement of the encoder and by a first spatial co-ordinate aligned at least substantially perpendicularly to the direction of movement of the encoder, which magnetically active areas are subdivided in a predetermined sequence along the second spatial co-ordinate, the arrangement comprising a magnetoresistive sensor element arranged in front of the main surface of the encoder in the direction of a third spatial co-ordinate aligned at least substantially perpendicularly to the main surface of the encoder, and a magnetic field-generating device for generating a magnetic field which permeates the magnetoresistive sensor element with field lines which are aligned at least substantially in a plane subtended by the first and the third spatial co-ordinate, the arrangement being adapted to generate a field line deflection, determined by the position of the encoder with respect to the magnetoresistive sensor element and the magnetic field-generating device, of the magnetic field generated by the magnetic field-generating device in the direction of the second spatial co-ordinate with a periodical recurrence determined by the predetermined sequence of the magnetically active areas and to supply an output signal having signal edges which rise and fall with said periodical recurrence and are produced by the magnetoresistive sensor element on the basis of the influence by this field line deflection, the arrangement being further adapted to generate a field line deflection, likewise determined by the position of the encoder, of the magnetic field also in the direction of the first spatial co-ordinate in said periodical recurrence, and a signal-processing circuit for detecting the slopes of consecutive signal edges in the output signal and for supplying a motion direction signal in dependence upon the values and/or the sequence of values of the slopes of consecutive signal edges.
The invention is based on the recognition that the characteristic curve of a magnetoresistive sensor element, which describes the dependence of an output voltage supplied by this element on the magnetic field strength of a component of the magnetic field generated by the magnetic field-generating device and occurring due to the field line deflection in the direction of movement of the encoder, has a slope which is dependent on the alignment of the magnetic field lines also in the direction of the first spatial co-ordinate, i.e. substantially perpendicularly to the direction of movement of the encoder. The smaller the alignment of the magnetic field lines also in the direction of the first spatial co-ordinate, i.e. substantially perpendicularly to the direction of movement of the encoder, i.e. the smaller the component of the magnetic field in the direction of this first spatial co-ordinate, the larger the slope of the characteristic curve in its zero region, which is determined by the disappearance of the magnetic field component occurring due to the field line deflection in the direction of movement of the encoder and the zero crossing of the output voltage supplied by the magnetoresistive sensor element. This means that, in the case of movement of the encoder along its direction of movement, i.e. in the direction of the second spatial co-ordinate, particularly steep changes of the output voltage supplied by the magnetoresistive sensor element are obtained at those areas where the magnetic field component in the direction of the first spatial co-ordinate is small, whereas the changes of the output voltage supplied by the magnetoresistive sensor element along the direction of movement of the encoder extend more flatly in those areas where the magnetic field component in the direction of the first spatial co-ordinate is larger. This involves a non-linear change of the characteristic curve along the direction of movement of the encoder, which has the effect that, in the case of motion of the encoder in dependence upon the direction of movement, the rising signal edges of the output voltage supplied by the magnetoresistive sensor element have a different slope than the falling signal edges of this output voltage. By evaluating these slopes of consecutive output voltage signal edges, i.e. edges rising and falling with said periodical recurrence, a motion direction signal indicating the direction of movement can then be directly obtained.
The magnetically active areas arranged along the main surface of the encoder and subdivided in a predetermined sequence along the second spatial co-ordinate may be formed differently, for example, as areas of a different magnetization of a magnetic material arranged along the main surface of the encoder or as toothed projections alternating with indentations along the direction of movement in a magnetizable material arranged along the main surface of the encoder.
The invention thus simply provides the possibility of determining the value and direction of the speed of a movement of the encoder with respect to the magnetoresistive sensor element. Advantageously, a conventional one-track encoder having only one row or track of magnetically active areas arranged along its main surface in the direction of movement, and only one magnetoresistive sensor element are required for this determination. The advantages such as, for example, the temperature compensation of such magnetoresistive sensor elements formed particularly as full Wheatstone bridges can thereby be utilized and the drawback of spatial mutual adjustments of a plurality of sensor elements can be avoided.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.